“This radical proposal basically kills the grandfather-concern addressed by both Congress and the administration and actually eliminates the appropriate review process failing to consider long-term readiness and retention outcomes in order to meet an arbitrary deadline so that Congress can go home for the holidays,” the groups wrote.

They are outraged that veterans and members of the military are footing $6 billion of the cost of the bill under the agreement, which lowers the cost-of-living adjustment for retirement accounts of service members younger than 62.

The veterans groups are trying to quickly mobilize opposition among their members over including military retirement in the budget deal, which provided $63 billion of sequester relief over the next two years in exchange — including restoration of $32 billion in automatic cuts to the Pentagon — for $85 billion in deficit reduction.

“We are not happy about it at all,” said Mike Barron, deputy director of government relations for the Military Officers Association of America (MOAA). “This was a backdoor deal in the worst kind of way.”

In their letter to congressional leadership, the groups say the retirement changes “will have a devastating financial impact for those who retire at the 20 year point by reducing retired pay by nearly 20 percent at age 62.”

“While portrayed as a minor change, a 20 percent reduction in retired pay and survivor benefit values is a massive cut in military career benefits and an egregious breach of faith,” the groups wrote.

“Ending the harmful effects of sequestration is a top priority for our nation’s security and military readiness, but to tax the very men and women who have sacrificed and served more than others is simply a foul,” they said.

The veterans groups said they were surprised by the last-minute inclusion of military retirement in the budget deal, and complained that the changes did not stem from the House and Senate Armed Services or Veterans Committees.

“During the press conference, they were talking of all the stuff they left off the table. I’m wondering why veterans and military retirees were on that table,” said Tom Tarantino, chief policy officer for Iraq and Afghanistan Veterans of America.

“This will negatively impact the military, and eventually impact national security,” he said. “How are you going to retain high quality people to spend a career in the military if they think their benefits will get cut?”

The veterans groups also complained that the commission enacted in the 2013 Defense authorization bill to study military retirement had specifically removed a “fast-tracking” rule to change military retirement benefits.

Several groups are trying to get their members involved to express opposition to including the military retirement reduction in the deal. Both Barron and Tarantino pointed to the 1980s attempt to cut military retirement benefits, known as REDUX, which was enacted by Congress but later repealed.

“VFW calculates that this will have a devastating impact on retirement benefits for retirees and survivors and jeopardizes the future value of military career benefits, and adversely effects recruitment, retention and our national security,” the Veterans of Foreign Wars (VFW) wrote in an alert to its members.

Still, it’s unclear what the organizations can do to remove the military retirement provisions from the budget deal, which the House plans to vote on Thursday.

The $6 billion portion from military retirement is a relatively small part of the $85 billion in total deficit reduction to pay for the bill. There is also another $6 billion taken out of retirement accounts for federal workers, down from an initial proposal of $20 billion.